Heretofore, a widely used magnetic recording medium type involved coated type media prepared by coating a non-magnetic support with a an organic polymer binder in which a ferromagnetic powder as magnetic material is dispersed, and then drying the magnetic coating composition. In recent years, with an increasing demand for high density recording, so-called metal thin film-type magnetic recording media, in which a ferromagnetic metal thin film as a magnetic layer is formed by a vapor deposition method such as vacuum deposition, sputtering or ion plating, or a plating method such as electric plating or electroless plating, have been receiving attention because their electromagnetic characteristics can be greatly increased.
Theoretical and practical requirements for magnetic recording media for high density recording include high coercive force and low film thickness. Further, metal thin film-type magnetic recording media can be easily decreased in thickness by an order of magnitude as compared with coating type magnetic recording media and are high in saturated magnetic flux density.
In particular, metal thin film formation by the vacuum deposition method provides great advantages because it does not need waste treatment as required in plating method, and its process is straightforward and a rate of deposition can be controllably increased. As a method of forming a magnetic layer having desirably high coercive force and squareness ratio by means of vacuum deposition, an oblique deposition method is known as described in, for example, U.S. Pat. Nos. 3,342,632 and 3,342,633.
However, problems associated with magnetic recording media having a ferromagnetic metal thin film include weather resistance, running properties and durability. Also, problems stem from the fact that the magnetic recording medium is brought into high speed relative movement with a magnetic head in the course of recording, reproduction or erasing of magnetic signals. Under such operating conditions, running should be carried out both smoothly and in a stabilized manner, and at the same time, inadvertent contact with the head, abrasion or breakage should be avoided. Under such circumstances, and in order to increase running properties and durability, the provision of a lubricant layer or a protective layer on the ferromagnetic metal thin film has been studied.
For example, one known protective layer is prepared by dissolving a thermoplastic resin, a thermo-setting resin, fatty acid, a metal salt of fatty acid, fatty acid ester, or alkyl phosphate perfluoropolyether-based compound, and then coating the resulting solution upon the metal thin film.
In addition, it has recently been developed to increase durability by using protective coating compounds having a branched perfluoroalkenyl group (JP-A-61-107528 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")).
However, the metal thin film-type magnetic recording medium prepared according to the above conventional practice fails to exhibit sufficiently satisfactory running properties under wide humidity range conditions ranging from low humidity to high humidity. In addition, its durability is nonoptimal particularly in a low humidity region.
Further, increasing the thickness of the protective layer or the lubricant layer is accompanied by reduction of electromagnetic characteristics due to loss of spacing between the head and the tape.
Accordingly, the industry has awaited more improvements in metal thin film-type magnetic recording media which overcome the above-mentioned drawbacks of the conventional arrangements.